4.7 引入NULL对象

【1】引入NULL对象范例

Book.h

#ifndef _BOOK_H
#define _BOOK_H

#include <string>
using namespace std;

class INULLOperator
{
public:
    virtual bool isNull() = 0;
};

class Book : public INULLOperator
{
public:
    Book();
    Book(int id, string name, double dprice);

    bool isNull();
    string getName();
    double getPrice();
    static Book* createNullBook();

private:
    int m_nID;
    string m_strName;
    double m_dPrice;
};

class NullBook : public Book
{
public:
    bool isNull();
};

Book.cpp

#include "Book.h"

Book::Book() 
{}

Book::Book(int id, string name, double dprice)
    : m_nID(id)
    , m_strName(name)
    , m_dPrice(dprice)
{}

bool Book::isNull() 
{ 
    return false; 
}

string Book::getName() 
{ 
    return m_strName; 
}

double Book::getPrice() 
{ 
    return m_dPrice; 
}

Book* Book::createNullBook()
{
    return (new NullBook());
}

bool NullBook::isNull()
{
    return true;
}

main.cpp

#include "Book.h"
#include <iostream>
using namespace std;

class BookService
{
public:
    Book* getBook(int nId)
    {
        if (nId < 0)
        {
            return Book::createNullBook();
        }
        return (new Book(nId, "Design Pattern", 100));
    }
};

void main()
{
    BookService *pBService = new BookService();
    Book *pBook = pBService->getBook(-1);
    if (pBook->isNull())
    {
        cout << "not found. " << endl;
    }
    else
    {
        cout << "name :: " << pBook->getName() << endl;
        cout << "price :: " << pBook->getPrice() << endl;
    }
}

【2】总结

当你需要再三检查某个对象是否为NULL。将NULL值替换为NULL对象。

Good Good Study, Day Day Up.
顺序 选择 循环 总结